This invention relates to a process for preparing silicon nitride powder having a high .alpha.-phase content and, more particularly, to a process for preparing silicon nitiride powder having a high .alpha.-phase content which powder is suitable as a raw material for the manufacture of high-density and high-strength sintered articles of silicon nitride.
In general, silicon nitride powder having a high .alpha.-phase content is suitable as a raw material for the manufacture of high-density and high-strength sintered articles of silicon nitride, because of its excellent sinterability and reactivity.
Conventionally, there has been the process for preparing silicon nitride powder, commonly called the "silica reduction process", which comprises blending silica powder and graphite powder at a stoichiometric ratio (C/SiO.sub.2) of 0.4 by weight and heating the blend in a nitrogen atmosphere whereby the active silicon-containing vapor resulting from the reduction of the silica powder by the graphite powder is caused to react with the nitrogen. However, the silicon nitride powder prepared by this silica reduction process has a low content of .alpha.-Si.sub.3 N.sub.4 and contains unreacted SiO.sub.2 or Si.sub.2 ON.sub.2, so that its quality is too low for sintering use.
On the other hand, the following two processes for preparing silicon nitride powder having a high .alpha.-phase content have recently been proposed. One of them is to heat metallic silicon powder in a nitrogen atmosphere while controlling the partial pressure of nitrogen gas whereby the silicon powder is directly nitrided at a temperature of 1300.degree. C. or below. The other is to bring silicon tetrachloride into contact with ammonia gas at a temperature of 1500.degree. C. or below. However, neither of these processes has practical utility partly because the reaction rate is too low to achieve a satisfactory production efficiency and partly because the equipment employed to effect the reaction is complicated and expensive.